Oil recovery process



United States Patent 3,027,942 OIL RECOVERY PROCESS Bertram Thomas Willman and George Wilfred Runberg, Tulsa, Okla., assignors to Jersey Production Research Company, a corporation of Delaware No Drawing. Filed July 2, 1958, Ser. No. 746,074 4 Claims. (Cl. 166-11) This invention is concerned broadly with the recovery of petroleum from subterranean reservoirs. It pertains more particularly to an improvement in the type of secondary oil recovery process in which steam is used as an oil driving medium. The invention has particular application in viscous oil reservoirs in which the oil in place has a viscosity about 50 centipoises or greater.

The production history of most petroleum reservoirs is considered to have two phases-a primary production phase and a secondary production phase. The primary production phase of a reservoirs history is that phase or period of time when oil is produced from the reservoir by means of a natural gas or water drive. Unfortunately, less than half of the oil within a reservoir can usually be obtained in this manner; and in the case of a reservoir containing a relatively viscous oil, recoveries of less than percent are not unusual. It is therefore necessary to resort to further techniques to recover additional quantities of the oil. It is in practicing these latter techniques that a reservoir experiences the secondary phase of its production history.

One of the techniques that has been suggested and tried for recovering oil from a reservoir during the reservoirs secondary production phase is one in which steam is injected into the reservoir through one or more injection wells. The steam, under pressure, eventually forces its way from the injection wells through the reservoir to the production wells; and, in so doing, it displaces oil from the reservoir.

In general, a steam drive method of secondary recovery is believed to operate somewhat as follows. First, the steam pushes its way into the reservoir and imparts heat to the reservoir. In giving up its heat, however, it condenses to form a bank of water which is followed by the main body of steam. Ultimately, then, any steam injection process becomes a process in which a bank of condensed water is propelled through a reservoir by means of a steam drive. The water and steam progressively heat up the reservoir and thereby materially reduce the viscosity of the oil in place. The resulting increased mobility of the oil enables it to be recovered from the reservoir in substantially increased quantities.

The use of steam drive processes for oil recovery appears to have its greatest attraction in those reservoirs which contain relatively viscous crudes--say SO-centipoise viscosity and greater. The ability of the steam to reduce the viscosity of the oil has a very marked and desirable elfect upon the amount of oil that can be recovered.

While the use of a steam drive in secondary recovery operations is known to have a number of advantages, several disadvantages have also been observed which greatly detract from its over-all value. A first disadvantage lies in the fact that steam-generation systems are very limited with respect to operating pressures. Thus, it is well known that pressures in excess of about 1800 to 2400 psi. are virtually impossible from either practical or economic standpoints.

The pressure limitation of steam-injection systems is very significant when it is recalled that the rate at which any fluid can be driven through a reservoir depends largely upon its driving force. Generally speaking, it is de sirable in any secondary recovery process to push the driving fluid through a reservoir at the greatest rate compatible with such factors as the overburden pressure and ice the amount of channeling and fingering. In the case of steam injection processes, the desirability of rapid rates of advance is especially pronounced, since great savings in heat are thereby possible. In this connection, it will be appreciated that substantial quantities of heat will be lost in any steam-injection process from the oil reservoir being treated to surrounding earth strata. Furthermore, the longer it takes for the steam to traverse a reservoir, the greater will be the quantities of heat lost in this fashion. It will be apparent, then, that steam-drive processes can be operated at maximum rates of steam travel only in relatively shallow reservoirs where the steam pressure is limited only by the overburden pressure. At greater depths, the available steam pressures may be much less than the pressures that could be tolerated by the reservoir. Thus, the steam-drive method of secondary recovery becomes progressively less attractive with deeper and deeper formations.

Having thus indicated the practical shortcomings of the steam-injection process for oil recovery, it is an object of this invention to reduce the effect of such shortcomings. More particularly, it is an object of the invention to make this process more attractive in all reservoirs-and especially in relatively deep reservoirs. It is a further object of the invention to reduce the quantities of heat lost in present steam processes and to thereby enhance the economics of the processes.

These and related objects of the invention may be realized by the technique of injecting water into a reservoir prior to the injection of steam. The amount, the pressure, and the temperature of the water may vary somewhat. However, the selection of these variables for any given reservoir will be quite readily apparent from the discussion that follows.

The purpose of injecting water into a reservoir prior to the injection of steam is to reduce the amount of time required for the steam to traverse the reservoir. By doing this, it becomes possible to save great quantities of heat which would otherwise be lost to surrounding formations. It will be apparent, of course, that the savings effected by this heat conservation must not be exceeded by the cost of performing the water injection. With this in mind, it is therefore desirable to inject the water at the greatest pressure compatible with overburden limitations. In other words, it is preferred to use as great a pressure as possible without causing damage to the reservoir structure, as by hydraulic fracturing or the like. In this way, other things being equal, the greatest possible rate of advance of the water may be attained.

With regard to the temperature of the water which is injected into a reservoir in the practice of this invention, it is preferred that the water be at the same temperature as the steam to be injected subsequently. In general, the greater the temperature of the water, the greater will be the reduction in the viscosity of the oil in place. Furthermore, the more heat that can be introduced within a reservoir by the injection of hot water, the less will be the heat required in the form of steam later.

Further relative to the temperature of the injected water, it should be noted that the mobility of any fluid within a reservoir is inversely related to its viscosity and directly related to the permeability of the reservoir toward the liquid. Accordingly, other things being equal, the mobility of the oil within a reservoir can be greatly increased simply by reducing the oils viscosity. In reservoirs containing viscous oils, it will further be recognized that viscosity reduction is a very significant factor in the rates and quantities of oil produced. Furthermore, the greater the viscosity of the oil, the greater will be the efiect of heat in reducing its viscosity. It is therefore desirable in practicing the present invention to employ water at its maximum possible temperature (consistent with liquid phase operation) in order to achieve full benefits of the invention.

Still another significant factor to observe in practicing this invention lies in the. amount of water which should be injected within a reservoir prior to the injection of steam. In this regard, it is desirable to inject a quantity of water such that the reservoir becomes sufficiently saturated with water so as to effect a substantial increase in the permeability of the reservoir toward water. The invention, in this respect, takes advantage of the fact that the relative permeability of a porous medium for any given fluid is directly related to the amount of that fluid contained within the medium. As noted earlier, the relative permeability of a porous medium toward a fluid directly affects the mobility of the fluid within the medium. Further, the rate of advance of a fluid through a porous medium (for any given driving pressure) is directly related to its mobility. Accordingly, it follows that the practice of this invention uniquely conditions an oil reservoir for the rapid and effective transmission of steam through the reservoir.

With regard'to the amount of water which should be injected within a reservoir prior to a steam drive, it is contemplated that water injection should be preferably terminated when the transmissibility or mobility of the water within the reservoir has increased to a point compatible with available steam-injection equipment. Data show that this point is ordinarily reached in a reservoir otherwise suitable for steam injection when enough water has been injected to increase the water content of the swept portion of the reservoir to a value of at least about 0.05 pore volume greater than the volume of connate water. In some instances, it may be found that the amount of water injected need only be about 0.05 pore volume to achieve this condition. In the ordinary case, however, the injection of about 0.25 to 0.5 pore volume of water will be required.

At this point, it is well to note that the process of this invention is applicable in general to any reservoir which is otherwise suitable for a steam drive process. In this connection, it is generally recognized in the art that reservoirs-to be suitable for a steam process-should be at least about 25 feet thick and should be highly permeable i.e., of the order of 150 millidarcies or greater. Furthermore, the reservoirat the time of steam injection-must be at a pressure less than available steam-injection pressure in order that steam may penetrate the reservoir. If a reservoir has a pressure greater than available steam pressure, it must first be reduced to a value below that of the steam. The pressure reduction may be effected prior K to or simultaneous with the water-injection step.

As explained above, it is necessary in the practice of this invention to inject suificient water prior to a steam drive in order to increase the water content of the reservoir at least about 0.05 pore volume greater than the connate water volume. Expressed otherwise, the amount of water injected should be sufiicient to create a mobile-as contrasted to a fixed-water phase. Actual detection of this condition may be achieved in several different ways. For example, it may be predetermined in the laboratory by experimentation with core samples derived =from the reservoir in question. It may also be determined practically within the field by observing the amounts and types of fluid that are produced from a reservoir as a result of the injection of the water. Thus, it may be generally assumed that the production of a given quantity of oil from a reservoir as a result of water injection is accompanied by a corresponding volume increase in the water content of the reservoir. In this connection, it should be noted that at the start of any water-injection program within a reservoir the reservoir may experience a certain amount of water fill-up as a result of voids or gases present therein. Generally speaking, negligible amounts of oil will be produced from a reservoir during the fill-up period.

To further illustrate the present invention, attention is directed to the example of a reservoir lying 3000 feet below the earths surface and possessing a pressure of 1500 p.s.i. The reservoir is penetrated by injection and producing wells arranged in any suitable pattern, and it possesses a permeability conducive to a steam-injection process. Available steam-injection equipment may be assumed to have an operating pressure of 1000 p.s.i. and a capacity of bbls. of steam (measured as condensate) available for each injection well.

In applying the present invention to the above reservoir, water is first injected into the reservoir through the injection wells and driven through the reservoir toward the producing wells. The water is preferably heated and injected at a temperature of no more than about 540 F.-i.e. the available steam temperature. It is also preferably injected at a pressure just below but approaching the formation breakdown pressure. The producing wells are preferably pumped down prior to injection of the water so as to create the greatest practical pressure differences and flow rates between the injection and producing wells.

When a mobile water phase has been established-as evidenced by such factors as increased water injectivity, oil production and produced fluid volumes-the injection wells are shut in. The water injectivity of the reser voir should also have increased to a substantial part of the capacity of the steam generation equipment at this point. Generally speaking this will have been accomplished after .05 to .25 pore volume of oil has been produced. The reservoir pressures are then allowed to drop below the available steam injection pressure (i.e. about 1000 p.s.i.), and steam injection is started. As is the custom, production wells continue pumping to obtain maximum steam injection rates.

In regard to the creation of a mobile water phase within the reservoir, it can be assumed that such a phase is present whenever the produced fluid volumes and the injected water volumes are approximately equal (fill-up having been accomplished). Steam injection following the creation of the mobile water phase may be carried out in a conventional manner.

As will be apparent, the benefits to be derived by practicing the present invention may vary somewhat from reservoir to reservoir. In the main, however, the invention enables the steam-injection process to be applied in many reservoirs where it was previously neither economical nor practical to do so. More specifically, it effects a substantial reduction in the exploitation time required for steam processes. In this connection, it is particularly effective in materially reducing the boiler time required in the usual steam processesreductions of about 30 percent being possible. Reduced boiler time is very important in view of attendant less maintenance costs, water treating costs, etc.

It is not expected that the amounts of oil ultimately recoverable by the practice of this invention will be materially greater than the amounts that are recoverable by conventional steam processes. However, it is important to note in this regard that the present invention recovers its oil in much shorter steam injection times and with greatly reduced quantities of steam and heat. The invention also permits steam drive processes to be applied to much deeper reservoirs (for a given maximum steam pressure) than has been heretofore possible.

What is claimed is:

1. In a method of recovering oil from a subterranean reservoir wherein steam is injected within the reservoir through an injection well at a predetermined pressure from a given source of steam and oil is produced from the reservoir through a spaced production well, the improve ment which comprises injecting water into the reservoir through said injection well prior to the injection of steam at a pressure approaching but less than the pressure required to fracture the reservoir, stopping the injection of water when the quantity injected is suflicient to render the transmissibility of water through the reservoir compatible with said source, lowering the pressure of the reservoir as necessary following the injection of said quantity of water and prior to the injection of steam to a value less than the steam-injection pressure.

2. A method as defined in claim 1 in which the temperature of the water injected within the reservoir is a maximum consistent with keeping the water as a liquid phase within the reservoir.

3. In a method of recovering oil from a subterranean reservoir wherein steam is injected within the reservoir through an injection well at a preselected pressure from a given source of steam, and wherein oil is produced from the reservoir through a spaced production well, the improvement which comprises the steps of injecting hot liquid water through said injection well into the reservoir prior to the injection of steam, said water being injected at a pressure approaching but less than the pressure required to fracture the reservoir and in a quantity suflicient to create a mobile water phase within the reservoir, and lowering the pressure of the reservoir as may be necessary prior to the injection of said steam to a value less than the preselected steam injection pressure.

4. A method as defined in claim 3 in which the temperature of the water injected within the reservoir is a maximum consistent with keeping the water as a liquid phase within the reservoir, and in which the volume of water injected is suflicient to increase the water content of the swept portion of the reservoir at least 0.05 pore volume greater than the volume of connate water.

References Cited in the file of this patent UNITED STATES PATENTS 1,237,139 Yeomans Aug. 14, 1917 FOREIGN PATENTS 511 768 Great Britain Aug. 24, 1939 

1. IN A METHOD OF RECOVERING OIL FROM A SUBTERRANEAN RESERVOIR WHEREIN STEAM IS INJECTED WITHIN THE RESERVOIR THROUGH AN INJECTION WELL AT A PREDETERMINED PRESSURE FROM A GIVEN SOURCE OF STEAM AND OIL IS PRODUCED FROM THE RESERVOIR THROUGH A SPACED PRODUCTION WELL, THE IMPROVEMENT WHICH COMPRISES INJECTING WATER INTO THE RESERVOIR THROUGH SAID INJECTION WELL PROIR TO THE INJECTION OF STEAM AT A PRESSURE APPROACHING BUT LESS THAN THE PRESSURE REQUIRED TO FRACTURE THE RESERVOIR, STOPPING THE INJECTION OF WATER WHEN THE QUANTITY INJECTED IS SUFFICIENT TO RENDER THE TRANSMISSIBILITY OF WATER THROUGH THE RESERVOIR COMTHE TRANSMISSIBILITY OF WATER THROUGH THE RESERVOIR COMRESERVIOR AS NECESSARY FOLLOWING THE INJECTION OF SAID QUANTITY OF WATER AND PRIOR TO THE INJECTION OF STEAM TO A VALUE LESS THAN THE STEAM-INJECTION PRESSURE. 